Generally, electromotive machines have a rotor and a stator. The rotor is wound with field windings, which are disposed in slots in the body of the rotor. The stator is wound with stator coils, which are disposed in slots in the body of the stator. In the case of a generator machine, when the rotor is rotated by an external source of mechanical energy, such as a steam turbine or a gas turbine, and an excitation current is provided to the field windings, electrical energy is induced in the stator coils.
Stator coils are generally constructed from a plurality of individual conductors referred to as strands. The strands are stacked together to form a larger conductor (or coil) capable of carrying high voltages and currents. In many stator coils, the strands are twisted into a weaved pattern rather than simply being stacked one on top of another. This weaving technique is generally referred to in the art as Roebelling and helps prevent the inner strands of a stator coil, which are closest to the rotor, from carrying more current (and generating more heat) than the outer strands, which are further from the rotor. Roebelling helps ensure that each strand carries a similar amount of current and generates a similar amount of heat.
Some stator coils include integral vent tubes to cool the strands. These types of stator coils are referred to in the art as inner-cooled coils. In inner-cooled coils, a plurality of vent tubes may be stacked on top of one another and sandwiched between two or more stacks of strands. A cooling gas (e.g., hydrogen or air) is then pumped through the vent tubes to transfer heat away from the strands.
There are a number of challenges associated with manufacturing inner-cooled stator coils. For example, after a stack of strands has been Roebelled, the top and bottom surfaces of the stack are no longer smooth. The surfaces have significant discontinuities caused by the Roebelling of the strands. These discontinuities make it difficult to apply an outer layer of insulation, referred to as ground-wall insulation.
Another challenge may be that a relatively large voltage differential can develop between the strands and the vent tubes in a stator coil while a generator is operating. If this voltage differential exceeds the dielectric strength of the insulation between the strands and the vent tubes, an electrical short will occur between the copper strands and the vent tubes, which can lead to circulating currents in the vent tubes and/or catastrophic damage to the stator coil.
Although various techniques have been proposed to alleviate such challenges, there is still a need for further improvements in stator coil configurations that provide increased protection against electrical shorts and an ability to grade voltages present in the stator coil to avoid overvoltage conditions that could lead to electrical discharge activity during operation of the electromotive machine, while also reducing the complexity and costs associated with manufacturing stator coils.